This invention is directed to a method for controlling the emission of noxious gases during the granulation of molten slags and in particular to the control of the emission of SO.sub.2 and H.sub.2 S during the granulation of molten blast furnace slags to prepare such slags for use as portland blast furnace slag cement.
Processes for granulating molten slags and in particular blast furnace slags to produce a product which can be used to produce portland blast furnace slag cement are known. In these processes, the molten slag is contacted with water. Large quantities of steam and other gases are evolved. The molten slags contain sulfur as sulfides in various amounts. The sulfides react with hydrogen (H.sub.2) and water (H.sub.2 O) during granulation and as a result a gas, H.sub.2 S (hydrogen sulfide) is formed and mixed with the vapors.
Apparently, H.sub.2 S in the vapors is then oxidized by the oxygen in the atmosphere to form SO.sub.2 (sulfur dioxide) and H.sub.2 O (water or steam). A portion of the H.sub.2 S in the vapors is not reacted, hence the vapors from the granulation of molten slags discharged to the atmosphere contain both H.sub.2 S and SO.sub.2. Because H.sub.2 S and SO.sub.2 are both noxious and corrosive, the amounts of H.sub.2 S and SO.sub.2 which can be discharged into the atmosphere are strictly regulated.
Processes in which the granulation of the slag and the cooling of the slag after granulation are performed in a closed system to prevent the venting of noxious SO.sub.2 and H.sub.2 S gases to the atmosphere have been devised.
One such closed process is described in U.S. Pat. No. 3,738,820 to F. Osborne, et al entitled "Method and Apparatus for the Processing of Molten Slag." In the process, the molten slag is introduced into an enclosure and is sprayed with water in the enclosure. The granulated slag is transported and cooled on conveyors in the enclosure. As the slag is transported, it and the noxious gases are sprayed with water until all evolution of gases ceases. It is contended that the noxious gases are thus removed from the vapors and are thus not vented to the atmosphere. Scrubbing the vapors with water may succeed in the removal of such gases from the vapors discharged into the atmosphere, however, the emission of the noxious gases is not suppressed. The apparatus is bulky and complicated. The process is time consuming and hence expensive thereby increasing the cost of producing granulated blast furnace slag.
Another process which attempts to overcome the increased costs of granulating slags in a partially closed system is described in U.S. Pat. No. 4,046,542 issued to Kees J. Schmidt entitled "Method and Device for Granulating Blast Furnace Slag." The patent contends that molten blast furnace slag is granulated by a powerful spraying system in an open explosion room. The granulated slag is transported from the explosion room to a dewatering tank provided with a dewatering bed. The process does not suppress the formation of the noxious gases SO.sub.2 and H.sub.2 S. The apparatus does provide a means for venting the vapors containing the noxious gases, H.sub.2 S and SO.sub.2 into the atmosphere through a high chimney above the level of the workers so that the noxious fumes are not a nuisance to the workers or surrounding inhabitants. While the apparatus may achieve the purpose for which is was intended, namely to discharge the noxious vapors above the level of the workers, the basic problem of suppressing the formation of SO.sub.2 and H.sub.2 S gases and venting quantities of the gases which are too small to be obnoxious to the surroundings are not solved. Noxious vapors are still discharged into the atmosphere and as such may pollute the environment.
It is an object of this invention to provide a method for controlling the emission of SO.sub.2 and H.sub.2 S during the granulation of blast furnace slag wherein the gases formed during granulation are substantially free from any detectable amounts of SO.sub.2 and the amount of H.sub.2 S vented to the atmosphere is not objectionable.